X-ray radiation sources are commonly used in radiation inspection systems for non-destructive inspection of objects. X-ray radiation may be generated in such sources by the impact of a beam of accelerated electrons on a high atomic number (“Z”) target material, such as tungsten or tantalum. The electrons are accelerated by a potential difference established across a chamber, referred to as an acceleration energy. The deceleration of the incident electrons by the nuclei of the atoms of the target material generates radiation, referred to as Bremsstrahlung. A collimator is provided to direct some of the generated radiation onto an object to be inspected and to form the generated radiation into a beam of a desired size and shape. One or more radiation detectors are provided to measure radiation transmitted through and/or scattered from the object. The body of the radiation source may also be shielded. In order to prevent radiation from escaping the radiation inspection system, shielding is also provided around the system as a whole.
Small objects, such as luggage and carry-on bags, are typically examined by radiation in the kilovolt range. However, radiation in the kilovolt range may not penetrate objects thicker than about 5 feet (1.52 meters), particularly if the object is filled with dense material. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width (thickness)) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting sidewalls, may be of comparable sizes as cargo containers, at least when supporting objects. The term “cargo conveyance” is used to refer to all types of cargo containers and comparably sized pallets (and other such platforms) supporting objects.
Higher energy radiation beams are required to penetrate through denser materials than less dense materials and through thicker materials than less thick materials. The low energies used in typical X-ray luggage and bag scanners, described above, are generally too low to penetrate through the much larger cargo containers, particularly those with widths or thicknesses of 5 feet (1.5 meters) or more. While the required energy level depends on the contents of the container and the width of the container, radiation in the megavolt range is typically required. 6 MeV to 10 MeV may be used, for example. 9 MeV is commonly used because it will penetrate through most cargo containers, regardless of the contents. However, high Z and medium Z metals commonly used in X-ray radiation sources, such as tungsten, tantalum, and molybdenum comprise stable isotopes having neutron production thresholds (the energy required to remove a neutron from a nucleus of an atom of the isotope) in a range of about 6 MeV to about 10 MeV. For example, the calculated neutron production thresholds for the stable isotopes of tungsten range from 6.191 MeV to 8.415 MeV. The calculated neutron production threshold for the stable isotope of tantalum is 7.651 MeV. The calculated neutron production thresholds for the stable isotopes of molybdenum range from 7.369 MeV to 12.667 MeV. Since 6 MeV to 10 MeV is a common range to examine cargo conveyances, neutrons are typically produced.
Because of their ability to absorb larger amounts of photons than lower atomic number metals per unit volume, high Z metals, such as tungsten and lead, are also typically used to shield the target and collimate the radiation beam. The stable isotopes of lead have calculated neutron production thresholds of from 6.737 MeV to 8.394 MeV. If the generated X-ray radiation used to examine the objects has an energy above the neutron production threshold of the shielding material and collimator, neutrons will also be produced.
Since neutrons may be harmful to people proximate the scanning system, thicker shielding may be required to prevent the escape of neutrons from the scanning system or the room containing the scanning system. This may increase the size and cost of the system. Concrete walls are commonly used to shield a room containing a scanning system, preventing or decreasing the amount of neutrons and X-rays that may escape the room. If space or other requirements prevent the use of concrete walls, then a multi-layer wall may be used. For example, a thick wall of polyethylene or borated polyethylene may be used as an inner layer to shield neutrons and lead or steel may be used as an outer layer to shield X-rays. The outer layer also shields gamma rays emitted by the polyethylene.
Varian Medical Systems, Inc., (“Varian”) Palo Alto, Calif., sells X-ray radiation sources for medical therapy supported by a rotatable gantry that also supports a detector array. The gantry, the source, and the detector comprise an integrated unit, which is sold under the trade name CLINAC®. The radiation source comprises a copper target and tungsten shielding. Copper generates sufficient X-ray radiation for therapeutic purposes, and is less expensive than tungsten. CLINACs® are available at 4 MeV, 6 MeV, 10 MeV and above. Copper has two stable isotopes, copper-65, with a calculated neutron production threshold of 9.910 MeV, and copper-63, with a calculated neutron production threshold of 10.852 MeV. Since in the 10 MeV and above models of the CLINAC® the acceleration energy is above the neutron production threshold of copper-65 and tungsten, neutrons are produced. The collimator comprises a combination of tungsten and lead, which will also generate neutrons. The tungsten shielding will generate neutrons, as well.
Efforts have been made to reduce neutron emission from X-ray sources used in medical therapy, such as radiation treatment. See, for example, Neutron Contamination from Medical Electron Accelerators, NRCP Report No. 79, National Council on Radiation Protection and Measurements, Bethesda, Md., pp. 59-60 (1995). It is said to be difficult to reduce the number of neutrons produced per useful photon rad of radiation, in the space available in existing sources. (Id.) It is noted that neutron emission may be reduced by absorbing unwanted neutrons in a medium Z material, such as iron, instead of tungsten or lead; but it is also noted that much more iron is required than tungsten or lead and there is insufficient space to take advantage of this reduction completely. (Id.)
Varian also sells a Linatron® series of X-ray sources that generate X-ray radiation in the range of 1-10 MeV. In these sources, the target is tungsten, typically in the form of a disk. A disk of copper is attached to the downstream side of the tungsten, of the electron beam, to dissipate heat and to act as a final electron stop for electrons passing through the tungsten target. The tungsten target is the primary source of X-ray radiation. It is believed that a small amount of X-ray radiation may be generated by the copper disk as well, by the electrons passing through the tungsten target. If the acceleration energy of the source is greater than the neutron production threshold of the tungsten, neutrons may be produced.